Heat a large nonstick skillet over medium-high heat. Add beef, onion, and garlic; cook, stirring frequently, breaking up meat with a wooden spoon as it cooks, about 5-7 minutes. Stir in crushed tomatoes, tomato sauce, salt, oregano, basil, and red pepper flakes; simmer 5 minutes to allow flavors to blend.
Meanwhile in a medium bowl, stir together ricotta cheese and 1 cup of mozzarella cheese.
Spoon 1/3 of beef mixture into a 5 quart slow cooker. Break 3 lasagna sheets in half and arrange over beef mixture, top with half of ricotta mixture. Repeat with another layer and finish with remaining 1/3 of beef mixture.
Cover slow cooker and cook on low setting for 4 to 6 hours. Remove cover, turn off heat and season to taste is desired.
In a small bowl, combine remaining ½ cup of mozzarella cheese and parmesan cheese; sprinkle over beef mixture. Cover and set aside until cheese melts and lasagna firms up, about 10 minutes.
